Ovarian hormones, such as estrogen (E), are thought to play a significant role in regulation of anxiety, perhaps via modulating the serotonin (5-HT) system emanating from the dorsal raphe nucleus (DRN). In support of this hypothesis, we have recently found that chronic E treatment in ovariectomized (OVX) rats increases the mRNA level of tryptophan hydroxylase-2 (TPH2; the rate-limiting enzyme for 5-HT synthesis and one of the key control points in regulation of the serotonergic system) selectively in the mid-ventromedial and caudal DRN and also decreases anxiety-like behavior in the open field. In particular, increased TPH2 mRNA in the caudal DRN was correlated with decreased anxiety-like behavior in the open field, whereas in the rostral DRN was correlated with increased anxiety behavior. These results suggest that E may decrease anxiety by increasing the synthetic capacity of 5-HT in the caudal DRN. The proposed study will examine the overarching hypothesis that E increases TpH2 mRNA in caudal DRN, and this in turn reduces anxiety. I will test this hypothesis by investigating the whether TPH2 mRNA knockdown or overexpression in the caudal DRN will block or mimic, respectively, the effects of E on anxiety-like behavior in the open field and the elevated plus maze. This study seeks to elucidate the role of the DRN 5-HT system in hormone regulation of anxiety behaviors. Most importantly, findings from this study have significant clinical implications for treatment of mood disorders in women. [unreadable] [unreadable] [unreadable]